1. Field of the Invention
The present invention relates to a color tone control method that adjusts the amount of ink dispensed to a printing press that is controlled by the results of multispectral measurements on printed articles, in particular, it relates to determining a transfer function to calculate the amount of ink dispensed based upon multispectral measurements, wherein the deviation from the target color is taken as the deviation in the multispectral output, and then the aforementioned calculation with the transfer function is used to calculate a corrected amount of ink to dispense as a means to control the color tone for printing press.
2. Description of the Related Art
Since the color reproducibility of printing presses varies according to a number of factors, in order to achieve the desired color tone in a print, it is imperative to use a color tone measuring apparatus to measure a color print and then adjust the color tone by adjusting the ink key, which controls the amount of ink dispensed by the printing press.
For example, in color management systems, in order to match the color reproducibility in individual devices such as printing presses and plate makers, one must determine the percentage dot area for the printing press and the color, for example, something corresponding to the color model as defined by the Commission Internationale de l""Eclairage (CIE L*a*b*) such as the color data called the xe2x80x9cprofilexe2x80x9d by the International Color Consortium (ICC) (hereinafter referred to as the xe2x80x9cprofilexe2x80x9d). Then based upon these elements, digital data must be converted on the upstream side in order to achieve a printed article with measured values that approximate the desired colors to thereby achieve a match to obtain the color reproducibility in a number of devices.
However, even if prints are made using this profile, in order to obtain the color tones exactly as desired, it is necessary to maintain the exact conditions during actual printing that were in place when the printing press""s profile was prepared. But sometimes conditions of color reproducibility changes for a printing press, and sometimes conditions under which characteristic data was obtained change during actual printing.
In other words, color reproducibility is affected by small changes in the swing roller pass that evenly applies ink to the ink roller, the movement of the water roller, the printing pressure applied by the rubber roller body, etc. Further there can be variations in the materials used in the inks and printing paper, differences among printing presses, differences in humidity, temperature and in the start time for the printing which can all affect reproducibility, and even if printed at standard concentrations, there are cases where halftones are incompatible.
Accordingly, even when a color management system such as described above is used to prepare printing plates appropriate to a printing profile, for example, as indicated in Japanese laid-open patent application 2001-47605, spectral reflection measurements are made on the color coordinate values (L*a*b*) on actually printed articles (for commercial printed articles), and the concentrations of the inks are calculated in order to control the dispensing of ink. The case is the same when color management systems are not used. Proof prints and test prints (OK sheets) are measured using spectro-reflectometers for commercially printed items and then computations of the ink density are used to control the amount of ink dispensed.
When the amount of ink dispensed is controlled using spectral reflections, and when the spectral reflections are made to a high degree of precision, it is possible to determine correspondingly accurate color coordinates. However, as the resolution becomes finer in order to obtain high precision measurements, it is necessary to increase the number of channels, and so doing causes each signal to be smaller, which intensifies the influence of noise. In addition, the multichannel processing requires a great deal of time, so much as to be difficult to implement on a commercial basis.
At this point, attention was focused upon the redundancy of the spectral reflection wave forms, and an approach involving the learning of these spectral wave forms and then predicting the wave form by using just a few measurements. To wit, the spectral reflection wave forms change smoothly, and by keeping the materials such as ink and printing paper constant, it is possible to learn the characteristics of the spectral reflections in advance, and then, using just a few channels, predict the wave forms and reproduce them to a high degree of precision. Examples of the use of such technology include U.S. Pat. No. 5,319,472, and Japanese laid-open patent application 1997-43058, 2000-333186, 2001-99710, etc.
To wit, in U.S. Pat. No. 5,319,472 discloses a correction method in which 4 or more narrow band filters are interchanged as a picture image signal is obtained by light receptor elements, and then with a black filter (light blocking filter) that is substituted for the foregoing narrow band filter, and using a white sheet in place of the image, the image signals read by each of the narrow band filters are corrected by the signals obtained using the black filter and white sheet, and then, a coefficient is applied to that output in order to obtain the original spectral reflection of the image.
Also, disclosed in Japanese laid-open patent application 1997-43058 is the use of a plurality of band pass filters when scanning original, with the resulting signal being analyzed by statistical methods to compute a classification spectrum for the colors used as a means of determining the classification of the original article.
The method disclosed in Japanese laid-open patent application 2000-333186, illuminates the article to be photographed (the original) with a specific light source, and then using a plurality of filters that transmit different wavelengths, produces output into a plurality of channels of differing spectral sensitivity, and then either a photograph is taken with black and white film, which has an approximately uniform spectral sensitivity in the visible light wavelength range, and the image is scanned, or an image signal is obtained for each filter using a CCD sensor at the imaging position to obtain a wavelength range signal. Then, from that information, a multichannel camera can be used to regenerate the spectral reflectance of the article that was photographed, which provides spectral wave forms for each pixel of the photographed image that can be converted into a control signal for various image reproduction methods.
Japanese laid-open patent application 2001-99710 discloses the photographing of a multi-band image using a variable wavelength filter, which is then used to estimate the spectral reflectance of the article that was photographed. Since the estimated spectral information takes place over a short period of time, the precision of the estimate is not degraded, and the reflectance for each channel of the multi-band image is converted in a pre-prepared table for the corresponding reflectance for brightness values obtained by photographing a known chart. Then the table is used to estimate, in a short period of time, the spectral reflectance of the object photographed based upon the brightness values.
However, with regard to U.S. Pat. No. 5,319,472, as well as with Japanese laid-open patent application 1997-43058, 2000-333186, and 2001-99710, the first, U.S. Pat. No. 5,319,472 relates to a correction method for the spectral reflectance, Japanese laid-open patent application 1997-43058 makes a color classification determination on the article being reproduced, Japanese laid-open patent application 2000-333186 photographs an image with a multichannel camera and then produces spectral wave forms for each pixel that can be converted into control signals for various image reproduction methods, and Japanese laid-open patent application 2001-99710, quickly estimates the spectral reflectance of an item, and without losing precision, produces estimated spectral information in a short period of time. However, none of these methods relate to printing presses.
Also, when spectral reflectance was used in the prior art for color tone control, as described above, color coordinate values (L*a*b*) and ink concentrations were computed based upon spectral reflectance, and the results were used to control the ink supply, but it required a great deal of time to compute the color coordinate values (L*a*b*) or the concentrations for each of the inks, and the method was further plagued by degraded precision due to the necessity of making the conversions from color coordinate values to ink concentrations multiple times.
Accordingly, the object of this invention is to provide a method for providing direct control of the ink supply for printing presses from multichannel measurement results.
To wit, the present invention comprises A Color tone control method for a printing press which incorporates an ink dispensing apparatus that can either electronically or mechanically vary the amount of ink dispensed, and which controls the color tone based on a printed item measured by a multispectral measurement means to control said ink dispensing apparatus, said method comprising steps of: obtaining an output of said multispectral measurement means from a plurality of said printed item which are printed while varying the amount of ink dispensed, by utilizing percentage dot area information in print editing or utilizing percentage dot area information measured from a printing plate for said printed item; determining a transfer function to calculate the amount of ink dispensed corresponding to the amount of change in the multispectral output by said output; and computing the amount of ink dispensed to be changed, based on the output deviation in said multispectral measurement means"" output from the target colors for a commercially printed item, and said percentage dot area information related to the target colors of the commercially printed item by using said transfer function in order to control the amount of ink dispensed by the foregoing ink dispensing apparatus.
To wit, as described above, since the spectral reflectance wave form changes smoothly, as long as the materials conditions, the inks, paper, etc., remain constant, pre-learning the spectral reflectance makes it possible to closely estimate the original wave forms using just a few channels, and to reproduce a printed item with a high degree of precision. However being able to reproduce the original wave form with a high degree of precision does not mean computing the amount of change required in the ink supply after computing the color coordinate values or ink concentrations from the spectral reflections, rather, it is possible to directly compute the ink dispensation amount from the measurement results. Thus, by precedently determining a transfer function, which is computed based upon the multispectral output changes that corresponded to the amount of change in ink dispensation, it is possible, by just inputting deviation in the output of the multispectral measurement means for the target colors and the percentage dot area information for the commercially printed item, to compute the changes in ink dispensation that are required without computing the changes in the color coordinate values or ink concentration, and to thereby accurately control the color tone without losing precision due to multiple conversions.
Also, according to the present invention, the transfer function is characterized by the configuration wherein the transfer function is determined for the information of each set of printing materials.
Thus, by obtaining the transfer function for each set of materials, printing inks and printing paper, it is possible to use any type of printing materials, and then quickly and accurately control color tones using just a small number of channels for measurement results.
Further, the transfer function is characterized by the configuration wherein the contribution rate is added to determine said transfer function, and said contribution rate includes the amount of the surrounding effects caused by the swing roller movement and amount of ink transfer by using the percentage dot area information in the surrounding area.
Thus, by adding to the transfer function, the contribution rate of effects from the surrounding area, which are caused by movement of the swing roller and the amount of ink transfer from the ink dispensing apparatus, etc., it is possible not only to incorporate the amount of ink dispensation into the transfer function, but also the aforementioned transfer elements of the ink to the printing plate to thereby allow the accurate computation of the amount of ink dispensation to achieve an even more precise control of the color tone.
Further, the number of channels for said multichannel measurement means is determined according to the number of colors to be used by the printing press, and the number of two-color chromatic color combinations.
Since color mixture in offset printing are accomplished by dot overlay rather than by blending inks to create mixed colors, a magnified view of color blended areas on the print will reveal places where there is only one ink present, places where both inks are present, and places where there is no ink at all (the color of the printing paper). As a result, estimating the spectral wave form in the blend areas from the spectral wave forms of the individual blended inks is a complex matter. Accordingly, the number of channels required is determined according to the number of colors used by the printing press, and by the number of two-color chromatic color combinations from those colors. For example, in the case where the four colors of yellow, magenta, cyan and black are used, eliminating the black (an achromatic color) from the 4 colors, there are the following two-color combinations of the remaining 3 colors: yellow-magenta, magenta-cyan, cyan-yellow, thus, a total of 7 channels would be required when the three combinations are added to the four ink colors. As an example of two-color printing, if only two colors such as red and black were used, only two channels would be required since only the red is a chromatic color. Determining the number of channels in this manner eliminates making superfluous measurements and allows the measurements to be performed at a high speed.
Also, the invention is characterized by the configuration wherein said transfer function to calculate the spectral reflectance or color coordinates is determined by said multispectral measurement means.
As described above, characteristic of the present invention is the ability to calculate the amount of ink to be dispensed without calculating the spectral reflectance or color coordinates, however, transfer functions for the spectral reflectance or color coordinates also may be determined exactly as described above. There are times when the spectral reflectance or color coordinates are used for evaluation purposes, and it is therefore desirable to additionally prepare transfer functions for finding these values.